What is Ocean Acidification?

Ocean acidification occurs because the ocean absorbs carbon dioxide (CO2) from the atmosphere. When CO2 reacts with seawater, the concentration of hydrogen ions (H+) increases as part of the carbonate buffer system, and more H+ ions leads to a decrease in the pH of seawater. Currently, the ocean is basic (meaning it has a pH similar to baking soda or toothpaste); however, with increased CO2, the ocean’s pH is likely to shift towards more neutral conditions. Many marine organisms need carbonate ions (CO32-) to build their shells (shellfish, corals, and marine plankton), and thus a continued decrease in pH will be devastating for these organisms.

An example of the effects of Ocean Acidification on marine life. Image credit: Climate Central.

Why the Atlantic Sea Scallop?

Due to their complex, multi-stage life cycles, mollusks rely on the chemistry of their environment for success. For instance, many studies have found that larvae are affected by acidification, either by death or deformed shell structures. In addition, predator-prey interactions, such as prey detection and shell thickness, may be affected by ocean acidification. It is important to understand how ocean acidification will affect commercially important mollusk fisheries. The US Atlantic sea scallop (Placopecten magellanicus) was chosen as a test species because it is a valuable wild-caught fishery (regularly worth $500 million yearly), and it is currently a sustainable fishery. Therefore, managers can examine potential long-term effects of environmental change on the fishery.

Dredge catch of Atlantic sea scallops during the 2012 federal fisheries resource survey in the Great South Channel off the Massachusetts coast. Photo credit: NOAA Fisheries/NEFSC

The details…

In this study, researchers developed a decision-support framework that encompasses the multiple factors affecting the fishery. They improved upon a pre-existing model called the integrated assessment model (IAM) for P. magellanicus to include 1) potential impacts from ocean acidification 2) climate scenarios that incorporate economic development 3) management practices and 4) future changes in fuel costs. Biological impacts to the fishery were defined as:

High (other impacts beyond growth and calcification, such as predator-prey interactions)

The economic development portion of the model incorporated scallop pricing and demand based on how economic change will alter peoples’ disposable income. Management practices were classified by four levels: 1) no-catch limits, 2) low management (annual catch limits), 3) medium management (low management plus additional reference points every 5 years) and 4) high management (medium management parameters plus permanently closing some regions to fishing activity). The last factor that researchers examined was fuel costs because fuel costs can be upwards of 80% of a scallop boat operating cost. Future fuel costs could be impacted by carbon taxation and are thus important to consider how these changes may impact scallop fishing.

What does this mean for the scallop fishery?

In the study, there were 256 possible outcomes for the success of the scallop fishery based on the aforementioned parameters. These outcomes ranged from no change in any of the parameters to the highest level of climate impacts, management and economic change. When ocean acidification and warming trends were not considered in the model, the future of the fishery remains stable. However, upon the addition of environmental change impacts to the model, stock concentration and landings of the sea scallop declined. Increases in atmospheric CO2 to levels greater than 900 ppm and a temperature increase of greater than 3°C caused stock and landings declines from 10-50% of end of century estimates. When growth rate impacts and increased mortality of small scallops (from predation) were included in the model, the result was a 10-30% decline in stocks and landings by 2050, and a 50-70% reduction in stocks and landings by the end of the century. The results from the model also showed a population shift in size distributions, with smaller scallops being more abundant with the “high” ocean acidification impact. This shift in population size has significant implications for the fishery because larger scallops produce more eggs, thereby increasing the population more so than smaller scallops.

What about management practices?

The highest level of management practice only somewhat reduces the scallop biomass declines from ocean acidification impacts. With the addition of a 10% closure, the biomass increased and lead to a population dominated by larger scallops. However, the biomass buildup does not last over time, and with ocean acidification impacts. The lack of biomass buildup over time suggests that a 10% closure might not be protective enough to sustain the fishery with future changes in carbonate availability.

Conclusions

The authors conclude that this study needs to be carefully considered because no previous studies to date have examined the effects of ocean acidification on P. magellanicus. The model relies on experiments with other scallop species, so generalizing with other species may lead to errors when estimating the population and fishery success under varying environmental impacts. Therefore, future studies, such as examining scallops potential to adapt to changing environmental conditions, are needed to further examine the impacts of ocean acidification on the US Atlantic sea scallop. Even though there are gaps with the model, understanding how climate change and socioeconomic change will affect the US Atlantic scallop fishery is important to prepare for mitigation efforts. Running the model with increased management practices slightly offset the impacts from environmental change impacts, suggesting a need to enhance the scallop fishery management. For a more comprehensive examination of ocean acidification impacts on the scallop fishery, the authors suggest laboratory experiments to gather empirical data for how the scallops may respond directly to increased acidification.

I am a first year PhD student in the Rynearson Lab studying Biological Oceanography at the Graduate School of Oceanography (URI). Broadly, I am using genetic techniques to study phytoplankton diversity. I am interested in understanding how environmental stressors associated with climate change affect phytoplankton community dynamics and thus, overall ecosystem function. Prior to working in the Rynearson lab, I spent two years as a plankton analyst in the Marine Invasions Lab at the Smithsonian Environmental Research Center (SERC) studying phytoplankton in ballast water of cargo ships and gaining experience with phytoplankton taxonomy and culturing techniques.